1. Field of the Invention
The present invention relates to a power supply module that includes a power supply circuit, such as a DC/DC converter, and to a mounting structure to mount the power supply module on a printed wiring board.
2. Description of the Related Art
A power supply module that is to be mounted as one component on a printed wiring board is typically required to be small and have high efficiency. For example, International Publication No. 2008/087781 discloses a power supply module that has been reduced in size by using a multilayer ferrite substrate.
In the power supply module disclosed in International Publication No. 2008/087781, a coil is formed by thick film printing inside the multilayer ferrite substrate and a chip component, such as a switching control IC, is mounted on an upper surface of the multilayer substrate.
It is difficult to make the direct-current resistance (Rdc) of a coil small when the coil is formed using thick film printing as disclosed in International Publication No. 2008/087781. Increasing the line width and the film thickness of coil conductors is an effective way of reducing the direct-current resistance, but there is a limit to the extent to which this can be done while reducing the size of the power supply module.
Furthermore, since wiring is formed of Ag inside a ferrite ceramic and then fired so as to become integrated with the ferrite ceramic, there is residual mechanical strain resulting from the different thermal expansion coefficients when the temperature falls after firing. Consequently, it is technically difficult to suppress the generation of cracks and other defects that reduce reliability.
In addition, when a multilayer substrate formed of a ferrite ceramic is thin, the multilayer substrate is more likely to break, and therefore, it is difficult to reduce the thickness of a multilayer substrate.